The present invention relates to a rotary power tool and, more particularly, to a rotary power tool having a device for detecting the torque of a rotatable shaft.
Assembling operations in general, e.g., driving screws into works is often implemented by an air driver, electric driver or similar rotary power tool in order to enhance efficiency. The problem wit this kind of tool is that the torque for driving, for example, a screw differs from one operator to anther operator. In light of this, there have recently been proposed rotary power tools provided with a torque detecting device for stabilizing the driving torque.
For example, Japanese Patent Laid-Open Publication No. 2-25754 (referred to as Prior Art 1 hereinafter) discloses a rotary power tool having an electric driver control circuit, a sample and hold circuit, a differential amplifier, a peak hold circuit, and a digital display. The electric driver control circuit outputs a reset signal when a reaction torque due to the start-up of a motor becomes zero. The sample and hold circuit takes in a torque output in response to the reset signal. The differential amplifier amplifies a difference in voltage between the two signals. The peak hold circuit holds a peak torque signal voltage represented by the output of the driver control circuit. The peak torque is displayed on the digital display. Japanese Utility Model Laid-Open Publication No. 2-110480 (referred to as Prior Art 2 hereinafter) teaches a rotary power tool including a torque load cell. The torque load cell is provided on the axis of the rear end of a drive section (body). The other end of the torque load cell is connected to the bottom of a tubular holder which rotatably supports the drive section (body) coaxially therewith. A torque detector detects a torque from the output of the load cell and displays it.
Prior Art 1 is effective if the tool starts rotating before the bit thereof mates with the recess of a screw. However, when the tool is held by hand to drive a screw, the reaction torque due to the start-up of the motor does not always become zero. In fact, the reaction torque, in many cases, does not become zero, as proved by actually recorded torque waveforms. For this reason, Prior Art 1 is not practical. In Prior Art 2, the torque detecting device is situated independently of the tool and holds the peak torque of the tool. This brings about a drawback that a reset switch for resetting the peak torque held has to be operated on the independent torque detector, increasing the number of assembling steps. In practice, even a delay of 0.1 second is critical when a great number of screws should be continuously driven into works.